Gregor Mendel is considered the father of modern genetics. Mendel crossed purebred pea plants with each other and studied the physical traits of their progeny, such as whether the seeds were smooth or wrinkled, green or yellow, produced red or white flowers, etc. In doing so, he realized that all genetic information an organism carries is passed down to the next generation in discrete units termed genes. Alternate versions of genes are termed alleles. The principle rule that genotype (the genetic makeup of an organism) dictates the phenotype (outward appearance) was discovered.
Mendel discovered that genetic information of higher organisms is carried in duplicate, with one copy of every gene from both parents. Genomes of higher organisms are therefore termed diploid. The two copies of the gene could code different information for the same gene (heterozygous) or two identical copies of the same gene (homozygous). Mendel also realized that when two different versions of the gene were carried, only one was expressed. The expressed gene is considered dominant and is represented on a Punnett square as a capital letter. The other gene that is carried but not expressed, termed recessive, is shown as a lower case letter on a Punnett square.
